Rechargeable batteries are essential components for consumer electronics, electric vehicle, and large grid energy storage. The state-of-the-art lithium ion batteries have high energy density and power density; however, their limitations lie in the high cost, low natural abundance of lithium, and safety issues related to dendrite formation. Efforts continue in the search for alternative electrode and electrolyte materials that are environmentally benign and of lower cost.
Multivalent batteries are batteries in which more than one electron is involved in the electrochemical conversion reaction. One example of a multivalent battery is a magnesium ion battery. Magnesium is one of the most abundant elements on earth, and is an attractive electrode material with a high theoretical specific capacity of 2205 Ah/kg and a high theoretical energy density of 3800 mAh/g. Because of its two valence charges, Mg has a specific volumetric capacity of 3833 mAh/cc, higher than that of lithium metal (2046 mAg/cc). Thus, upon oxidation, magnesium can provide one electron to advance to the Mg+, which, in turn, may then provide a second electron to form Mg2+.
Although rechargeable, multivalent batteries have been studied for more than a decade, they are still facing several obstacles. For example, in terms of magnesium batteries, reaction between magnesium and standard electrolytes results in passivation of the magnesium surface. Unlike lithium ions, which can move through a solid electrolyte interface containing inorganic lithium salts (e.g., lithium carbonate and lithium fluoride), magnesium ions cannot pass such passivated films. Additionally, there is a need for safer and more efficient magnesium electrolytes. Other multivalent batteries, such as calcium ion batteries and aluminum ion batteries, face similar problems.
Magnesium batteries have a magnesium anode, a cathode, and an electrolyte to carry electrons from the magnesium to the cathode. Good ionic conductivity is required at the interface of the anode/electrolyte and the cathode/electrolyte. The anode/electrolyte interface requires a fresh surface that does not block the ionic conductivity. Grignard reagents (R—Mg—X, where R is an organic residue and X is halogen) are one type of electrolyte for magnesium batteries that provides for a good anode/electrolyte interface. For magnesium batteries, ether solutions of magnesium-based Grignard reagents allow reversible magnesium deposition and dissolution with a high coulombic efficiency. However, they have fairly low anodic stability, and exhibit an electrochemical window of less than 1.8 V. See Lossius, L. P. et al. Electrochimica Acta, 41(3): 445-447 (1996).
Typical Grignard reagents include those denoted by the formula RMgCl (where R=methyl, ethyl, butyl). By introducing a Lewis acid to the Grignard reagent results in a Grignard-Lewis acid, the electrochemical window of the resulting magnesium-Al electrolyte can be substantially increased, compared to ethyl Grignard (Et—Mg—Cl). See e.g. Aurbach, D. et al. Nature, 407(6805):724-727 (2000); Mizrahi, O. et al. J. Electrochem. Soc. 155(2):A103-A109 (2008); Guo, Y. S. et al. Energy & Environmental Science 5(10):9100-9106 (2012); Muldoon, J. et al. Energy & Environmental Science 6(2):482-487 (2012).
The current state-of-the-art intercalation electrode for magnesium ion batteries is Mo6S8, which is conventionally used in a THF (tetrahydrofuran) solution of a Grignard reagent, Mg(AlCl2EtBu)2, to construct a rechargeable magnesium ion battery. The Mg(AlCl2EtBu)2 electrolyte has an improved electrochemical stability up to 2.4 V vs Mg/Mg2+. However, despite the 100% efficiency of deposition/dissolution toward the magnesium electrode, the Mg(AlCl2EtBu)2 electrolyte is highly flammable and has a relatively low solubility in THF solution. The large molecular weight of Mg(AlCl2EtBu)2 also makes it less attractive as an electrolyte. Recent patents reported by Pellion Technologies Inc. include the MgaZbXc complexes (WO 2013/096827 A1), while Z and X form Lewis acid. For example, a THF solution of 2MgCl2/AlCl3, shows reversible magnesium deposition/dissolution with a electrochemical window of 3.0 V.
Another electrolyte with high oxidative stability is a THF solution of magnesium bis(trifluoromethylsulfonylimide) and magnesium chloride (Mg(TFSI)2/MgCl2). Such an electrolyte avoids the use of Grignard reagents. See U.S. Pat. Publ. 2013/0025211. However, the MgCl2/AlCl3 mixture utilizes strong Lewis Acid of AlCl3 which showed reactivity toward cyclic ether solvent (tetrahydrofuran), while the Mg(TFSI)2/MgCl2 can potentially have decomposition from the TFSI anion on the surface of magnesium anode.